Pressure sensitive adhesives (PSAs) are permanently tacky at room temperature and adhere to surfaces by application of light finger pressure. PSA compositions are commonly applied to various substrates, such as paper, fabric, metal, and plastic films that are then converted into a large number of different products, especially pressure sensitive adhesive tapes, medical tapes and labels. These pressure sensitive adhesive products have a broad field of application in the automobile industry, e.g., for fastening or sealing, in the pharmaceutical industry, e.g., for bandages or transdermal drug delivery systems, or in the packaging industry, e.g., for sealing, bonding or labeling.
PSAs can be formulated for application as a solvent borne or a molten adhesive. Hot melt pressure sensitive adhesives (HMPSAs) are compositions that combine the properties of hot melt adhesives with those of pressure sensitive adhesives. Hot melt adhesives are solids at room temperature, melt at elevated temperatures to coat on a substrate, and regain their solid form on cooling. The combination of these properties provides compositions that melt at elevated temperatures and cool to form a permanently tacky solid coating that adheres on contact. A good workable HMPSA must exhibit high cohesive strength at room temperature, low shrinkage on substrates, retention of pressure sensitive properties during storage and use, and a relatively fluid viscosity at typical coating temperatures (e.g., between 80° C. and 180° C.). Although very low molecular weight polymers will yield hot melt adhesives with sufficient fluidity, the resulting adhesives lack cohesive strength. Very high molecular weight polymers give better cohesive strength, but are too viscous at the common application temperatures to be easily coatable on substrates. They must be blended with a high proportion of low molecular weight oils or resins to reduce the viscosity. The addition of low molecular weight oils or resins in turn detracts from the cohesive strength and heat resistance. To avoid these problems, polymers of moderate molecular weight have been made with various functional groups which undergo crosslinking reactions (curing) by heat or actinic radiation. In this manner, the cohesion of acrylic PSAs can be raised by means of sufficient curing.
Acrylic polymers are widely used in the field of PSA, because they offer great synthetic versatility in that a wide range of monomers are commercially available and the polymer architecture can already be controlled during polymerisation.
However, the technologies for crosslinking acrylic polymers known in prior art have some drawbacks.
Some technologies in prior art require the addition of multifunctional polyol and other hydroxyl-functional groups for crosslinking the polymers.
Some technologies in prior art are directed to the use of photopolymerizable diacrylate. Due to the difunctional acrylate which crosslinks upon radical polymerization, it is less desirable for use in an adhesive, and especially unsuitable for a hot melt adhesive, because it is necessary to polymerize the acrylate monomers following application onto the final coated substrate.
With increasing pressures on reducing environmental emissions from factories and energy costs, there is an ongoing demand and a continuing need in the art for UV-curable acrylic polymers that are hot melt processable. The current invention addresses this need by providing hot melt compositions comprising UV curable acrylic polymers and cationic photoinitiators and, following the coating operation, are cured under UV irradiation on the substrates. The cured product is a hot melt acrylic PSA.